Ferrous metal substrate with dense, black glossy oxide coating and process for coating preparation

ABSTRACT

A composite structure is described in which a substrate is covered with an adherent protective/ornamental layer consisting of single phase cobalt oxide containing iron in solid solution therein. By way of example, a ferrous metal substrate to which a layer (at least about 0.5 mil thick) of cobalt has been applied is treated to provide a black, dense, coherent scale possessing a deep luster and high reflectivity. This enamellike coating is produced by (a) heat treatment of the plated substrate in an atmosphere not oxidizing to either cobalt or iron in order to interdiffuse the cobalt and iron and produce a cobalt-iron surface composition in which the iron content may range from about 2 to about 20 percent by weight and (b) oxidation of the plated substrate after it has been heated to temperatures in the range of 900* to 1,200* C.

Unite States Watent Tedmon, .lr'. et all.

[ 1 Feb. 1, 1972 [72] Inventors: Craig S. Tedmon, Jan, Scotia; ll-llenry S.

Spacil, Schenectady, both of NY.

221 Filed: Mar-17,1969

21 Appl.No.: 811 213 [52] 11.8. C1 .148/63, 117/107, 148/6, 148/635, 148/315 [51] Int. Cl ..C23c 117/00 [58] Field of Search ..117/49, 50, 107.; 148/6, 6.3, 148/635; 75/170 [56] References Cited UNlTED STATES PATENTS 3,012,904 12/1961 Baer et a1 ..117/50 3,055,089 9/1962 Drummond. ....l17/50 X 3,091,022 5/1963 Faulkner ..75/ 170 X Primary ExaminerRalph S. Kendall Assistant Examiner-Caleb Weston AtrorneyRichard R. Brainard, Paul A. Frank, Charles T. Watts, Leo l. Malossi, Frank L. Neuhauser, Oscar B. Waddell and Joseph B. Forman [57] ABSTRACT A composite structure is described in which a substrate is covered with an adherent protective/ornamental layer consisting of single phase cobalt oxide containing iron in solid solution therein. By way of example, a ferrous metal substrate to which a layer (at least about 0.5 mil thick) of cobalt has been applied is treated to provide a black, dense, coherent scale possessing a deep luster and high reflectivity. This enamellike coating is produced by (a) heat treatment of the plated substrate in an atmosphere not oxidizing to either cobalt or iron in order to interdiffuse the cobalt and iron and produce a cobaltiron surface composition in which the iron content may range from about 2 to about 20 percent by weight and (b) oxidation of the plated substrate after it has been heated to temperatures in the range of 900 to 1,200 C.

3 Uaims, No Drawings BACKGROUND OF THE INVENTION US. Pat. No. 1,022,29l-Axelrod discloses a process for preparing a firmly adherent brown, steel-gray or blackcolored coating on the surface of "any metals forming colored compounds with cobalt salts." Briefly, the metal article (with or without a surface oxide film) is covered with a solution (pl-I 7 or greater) of a cobalt salt and the article is then heated. The extent and rate of heating determine the shade produced and the particular color produced depends upon the specific cobalt oxide(s) formed.

The surface produced by the Axelrod process is materially different in several respects from the instant invention. Thus, in the Axelrod process oxides are reacted together (the oxide of the base metal is reacted with cobalt oxide) while the instant invention does not involve any reaction of or between oxides, but rather a formation of cobalt oxide into which the iron dissolves and remains in solid solution. Procedurally, the Axelrod patent refers to a single heating operation of a substrate covered with a solution of cobalt salt, which heating must be presumed to be in an oxidizing atmosphere. With the instant invention, on the contrary, either a cobalt/iron alloy is heated in an oxidizing atmosphere at preselected temperatures or a layer of cobalt/iron alloy is prepared in situ and then oxidized at preselected temperatures. Preparation of the cobalt/iron alloy in situ is carried out by depositing cobalt metal over a ferrous metal substrate and then heating in an atmosphere, which is not oxidizing to either cobalt or iron. Further, the thickness of the Axelrod coating cannot exceed about 1 micron due to the method of application, while the thickness of the coating in the instant invention must be greater than at least one-half mil.

In US. Pat. No. 1,779,273-Hommel the surface of an iron or steel article is treated so as to produce a mixed oxide surface layer (iron oxide plus an oxide of cobalt, nickel or manganese). The maximum temperature to which the article is subjected during heat treatment (prior to subsequent enamelling) is about 760 C. Undoubtedly, a multiphase layer is produced by this reaction between oxides under oxidizing conditions and this layer is materially different from the single-phase enamellike coating in the instant application. This becomes apparent from the fact that the oxide film layer of Hommel simply provides a base for a later-applied material for providing an enamel.

Sutphen et a1. (U.S. Pat. No. 2,775,210) discloses the preparation of a scale on the surface of a ferrous meta] substrate. The metal substrate is coated with a thin layer of cobalt (or nickel) so that the minute isolated patches of the iron substrate are protected by cobalt during subsequent heating of the metal substrate in an atmosphere oxidizing to iron but not oxidizing to cobalt. After this oxidation step, the surface consists of tiny patches of iron oxide interspersed with patches of cobalt metal. In the coating of the instant invention, on the contrary, oxidation of the cobalt containing minor amounts of iron atoms results in a crystalline material identifiable as cobalt oxide containing iron in solid solution therein, there being no discernible iron oxide phase.

SUMMARY OF THE INVENTION A cobalt oxide layer containing from about 2 to about percent iron dissolved as iron in the cobalt oxide (C00) phase and diffused therethrough provides an adherent black, dense, lustrous attractive, highly reflective surface. On a ferrous metal substrate, for example, this coating is produced by applying a layer at least about 0.5 mils thick of cobalt metal to the substrate; heat treating the plated substrate in an atmosphere not oxidizing to either cobalt or iron to bring about interdiffusion of the cobalt and iron resulting in the requisite cobalt-iron surface layer composition and then oxidizing this cobalt-iron surface layer at temperatures in the range of 900 to l,200 C. to convert the surface composition to a crystalline structure identifiable as a solid solution of iron in cobalt oxide, there being no discernible iron oxide phase therein.

DESCRIPTION OF THE PREFERRED EMBODIMENT Although the instant invention is more readily applied to a ferrous metal substrate, as for example iron, steel, ferroalloys, etc., any substrate, which will provide the necessary support and sustain long periods of exposure to oxidizing and/or nonoxidizing atmosphere at temperatures in the range of 900 to l,200 C. and over which the requisite cobalt-iron surface composition may be applied, as by codeposition of the iron and cobalt, may be used. Codeposition may be accomplished by electrodeposition or plasma spraying, for example.

Manifestly, the coating of this invention may be produced on the surface of bodies of the cobaltiron alloy composition described herein.

Since, in each of the above cases, the cobalt-iron composition would already be preexisting at the time of the oxidation, oxidation of the surface layer would be the only step necessary, however, all oxidation must (as in the preferred method set forth below) be carried out with the temperature of the cobalt-iron composition above 900 C. and preferably in the 900-l,200 C. range. This may be best accomplished by preheating the body in a nonoxidizing protective atmosphere and then introducing the desired oxidizing atmosphere into contact with the heated body.

Preferably, the requisite cobalt-iron surface composition is prepared in situ and then subjected while at temperatures in the range of 900 to l,200 C. to an oxidizing atmosphere. This formation of a cobalt-iron surface layer composition is accomplished by applying a thin layer (about 0.5 mils to 5 mils thick) of cobalt metal over a ferrous metal substrate utilizing well known electroplating (or electroless plating) techniques. The substrate so plated is then heated in an environment, which is nonoxidizing to either cobalt or iron (e. g., in an inert atmosphere with or without a reducing gas component or in a vacuum) at some temperature in the range of from 800 to l,450 C. for the purpose of bringing about interdit'fusion between the cobalt and iron atomssufficient to raise the iron content of the cobalt layer up to about 2-20 percent by weight and, if necessary, subsequently changing the temperature of the plated substrate to the preselected temperature range to conduct the oxidation step (between about 900 and l,200 C.). Next, an oxidizing atmosphere is introduced, while keeping the temperature within the above preselected range until the surface layer has been oxidized. Thereafter, the coated substrate is cooled to room temperature in the oxidizing environment, or in air, whereupon it is found that the surface of the substrate has an adherent, attractive, black, glossy, highly reflective enamellike coating.

If less than about 2 percent by weight of iron is dispersed in the cobalt, when the oxidation step of this invention is carried out, the surface usually displays a dull matte finish as would be the case with pure C00. With greater than about 20 percent by weight of iron in the cobalt, when the oxidation step of this invention is carried out, a second phase appears, the surface is less smooth and attractive, and the oxide layer is inclined to be less adherent.

In preparing the desired cobalt-iron composition the in situ heating in the nonoxidizing atmosphere should be at a temperature in excess of l,O00 C. to achieve about 15 percent Fe by weight and in excess of l,100 C. to achieve about 20 percent Fe by weight. At concentrations of Fe greater than about 20 percent by weight a second, unwanted phase develops, because the solubility of Fe in the C00 will have been exceeded. The amount of iron actually introduced may be readily determined by securing X-ray diffraction patterns for the material, measuring the lattice parameters displayed therein and comparing the measured lattice parameters with a graph showing the variation of lattice parameters from those lattice parameters for pure C as various amounts of iron become dissolved therein.

A specific example illustrating the above process is as follows:

a. A pure iron plate was mechanically polished with No. 600

grit silicon carbide paper,

b. a coating of cobalt 0.002 inch thick was electrodeposited on the iron plate and c. the cobalt-coated plate was heat treated according to the following schedule:

1. the plate was heated in vacuum to 500 C. and held at this temperature for a period of about 15 hours to remove any moisture or other entrapped volatiles;

2. the temperature of the coated plate was raised to about l,000 C. and held for 8 hours (still under vacuum) to effect the requisite interdiffusion and 3. still holding the temperature at 1,000 C. air was introduced for a heating period of about an hour during which the surface oxidized and 4. the coated plate was cooled to room temperature in air.

Xray diffraction of the surface layer identified the crystalline material present in this layer as cobalt oxide containing iron dissolved therein and metallographic examination established that no second phase (iron oxide) was present.

The black-glossy, enamel-type oxide surface layer so produced is abrasion resistant (i.e., to scratching by metals) and may be employed to provide an attractive, easily cleaned surface for such items as oven liners, jewelry, lamp reflectors, etc. Also, because of the high electrical resistance of this surface layer at room temperature, this material has application as an electrical insulator.

These surface layers should not be subjected to extensive exposure to temperatures in the range between about 400 C. and 900 C., because in this range the composition exhibits instability. After such exposure the surface layer composition degrades such that the surface develops a dull, matte finish and cracks may develop. However, the kinetics of this conversion are such that at temperatures below about 400 C. this problem of conversion to the unstable form is not a serious one.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. In a composite structure wherein a substrate is covered with an adherent protective/ornamental layer, said substrate providing support for said layer during and after formation thereof and being able to sustain extended exposure to oxidizing and/or nonoxidizing atmospheres at temperatures in the range of 900l,200 C., the improvement wherein the protective/ornamental layer is a black, lustrous single phase metal oxide consisting of cobalt oxide (C00) containing iron in solid solution therein, said protective/ornamental layer being at least about 0.5 mil in thickness and the iron content thereof being between about 2 percent and 20 percent by weight.

2. The improvement of claim 1 wherein the substrate is a ferrous metal.

3. The method of preparing a protective/ornamental layer over a ferrous surface comprising the steps of:

a. depositing a layer of from 0.5 to 5 mils of cobalt metal over a ferrous substrate,

b. heating the coated substrate at a temperature in the range of from 800 to l,450 C. in a gaseous atmosphere nonoxidizing to either said substrate or said cobalt layer for a period sufficient to bring about the desired interdiffusion of iron and cobalt,

maintaining the modified substrate and iron-enriched cobalt layer at temperatures in the range of from 900 to l,200 C. and admitting an oxidizing atmosphere into contact therewith for at least about 1 hour and a d. permitting the oxide-coated substrate to cool. 

2. The improvement of claim 1 wherein the substrate is a ferrous metal.
 3. The method of preparing a protective/ornamental layer over a ferrous surface comprising the steps of: a. depositing a layer of from 0.5 to 5 mils of cobalt metal over a ferrous substrate, b. heating the coated substrate at a temperature in the range of from 800* to 1,450* C. in a gaseous atmosphere nonoxidizing to either said substrate or said cobalt layer for a period sufficient to bring about the desired interdiffusion of iron and cobalt, c. maintaining the modified substrate and iron-enriched cobalt layer at temperatures in the range of from 900* to 1,200* C. and admitting an oxidizing atmosphere into contact therewith for at least about 1 hour and d. permitting the oxide-coated substrate to cool. 